Monochromators are generally well known for use in the energy range from a few kilovolts and up. These monochromators generally use single crystals of silicon or germanium as energy dispersive elements. An article by J. A. Golovchenko of Bell Laboratories printed in the Review of Scientific Instruments, Vol. 52, No. 4, April 1981, describes a double crystal monochromator wherein each crystal element is changed by the experimenter to obtain a constant output direction as well as a constant beam position as energy is varied and to make the central ray of the selected beam impinge at the same point on all monochromator crystals independently of the chosen energy. This was achieved by sampling the output beam intensity for angular drifts out of parallelism between two crystals with an electronic drive to correct the beam.